Journal of Materials Chemistry C

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Supramolecular design principles for efficient photoresponsive polymer–azobenzene complexes Cite this: J. Mater. Chem. C, 2018, 6,2168 Jaana Vapaavuori, *ab C. Geraldine Bazuin *b and Arri Priimagi *a

Noncovalent binding of azobenzenes to polymers allows harnessing light-induced molecular-level motions (photoisomerization) for inducing macroscopic effects, including photocontrol over molecular alignment and self-assembly of block copolymer nanostructures, and photoinduced surface patterning of polymeric thin films. In the last 10 years, a growing body of literature has proven the utility of supramolecular materials design for establishing structure–property–function guidelines for photo- responsive azobenzene-based polymeric materials. In general, the bond type and strength, engineered by the choice of the polymer and the azobenzene, influence the photophysical properties and the Received 2nd November 2017, optical response of the material system. Herein, we review this progress, and critically assess the Accepted 22nd January 2018 advantages and disadvantages of the three most commonly used supramolecular design strategies:

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. DOI: 10.1039/c7tc05005d hydrogen, halogen and ionic bonding. The ease and versatility of the design of these photoresponsive materials makes a compelling case for a paradigm shift from covalently-functionalized side-chain rsc.li/materials-c polymers to supramolecular polymer–azobenzene complexes.

1. Introduction 1.1. Supramolecular functionalization Design and fabrication of functional materials based on covalent

This article is licensed under a chemistry has been an important topic in synthetic chemistry for a Laboratory of Chemistry and Bioengineering, Tampere University of Technology, more than a century. In contrast, it is only a few decades ago that P.O. Box 541, FI-33101 Tampere, Finland. E-mail: [email protected] supramolecular chemistry, which exploits noncovalent bonds, b De´partment de Chimie, Universite´ de Montre´al, C.P. 6128, Succursale Centre-Ville, was established as a distinct field by Jean-Marie Lehn.1–4 Despite

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. Montre´al, QC, H3C 3J7E, Canada. E-mail: [email protected], [email protected] being a relatively young field, supramolecular chemistry has

Jaana Vapaavuori defended her Geraldine Bazuin, who obtained her doctoral thesis, ‘‘The Supra- PhD at McGill University in 1984 on molecular Design of Efficient the subject of ionomers, followed by Photoresponsive Materials’’, done a postdoc in Strasbourg, France in under supervision of Dr Arri the area of liquid crystals, was Priimagi and Prof. Matti Kaivola, professor first at Universite´ Laval in the department of Applied from 1986 to 2003, then at Physics at Aalto University, Universite´ de Montre´al since 2003. Helsinki, Finland, in 2013. She led one of the pioneering Currently, she is a Banting research groups working with Postdoctoral Fellow in the supramolecular side-chain liquid Department of Chemistry at the crystal polymers, focusing in Jaana Vapaavuori University of Montreal, Canada C. Geraldine Bazuin particular on ionically bonded working together with Prof. systems. Recently, this work has Geraldine Bazuin and Prof. Christian Pellerin. Her main research included azo-containing small molecules and has embraced interests involve developing new materials and design concepts for (H-bonded) supramolecular block copolymer systems, particularly in light-controllable nanostructures and liquid crystals, for all-optical the form of dip-coated and Langmuir–Blodgett thin films. surface patterning of polymers, and for electrochemical devices.

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become a very successful tool and has given rise to many The supramolecular functionalization strategy is flexible, subfields, such as molecular self-assembly, molecular recognition, facile and cost-effective, simplifying the preparation of libraries template-directed synthesis, mechanically interlocked molecular that isolate particular molecular parameters, such as the architectures, biomimetics, and molecular machinery.5 In its chromophore dipole moment, while keeping other parameters, simplest expression, two molecules having complementary such as polymer backbone molecular weight and polydispersity interaction sites at mutually accessible locations join together index, constant. Furthermore, it can be applied to a variety of host non-covalently to form a supramolecule.5 The variety and architectures, including linear, star-like and dendritic homo- strengths of feasible supramolecular interactions is vast, polymers and block copolymers, having the tendency to form ranging from strong ionic bonds with interaction strengths as self-assembled periodic patterns at the scale of 10–100 nm,13 high as 350 kJ mol1 to very weak hydrogen bonds with strengths the formation of which can be controlled by block-selective less than 5 kJ mol1. Hence, depending on the strength of the attachment of azobenzenes. interaction, a spectrum of different material properties can be obtained, where the weaker bonds can give an equilibrium of 1.2. Azobenzene at work: photoinduced anisotropy and bonded and non-bonded states and the strongest ones have surface patterning covalent-like character.6,7 Azobenzene derivatives have been popular building blocks in In the present review, focusing on recent developments in photoresponsive supramolecular materials due to their clean supramolecular photoresponsive materials, the latter are fabri- photoisomerization reaction, consisting of a photoinduced cated through ionic, hydrogen and halogen bonding between reversible switching of the azobenzene geometry between the generally photopassive host materials and photoactive azobenzene rod-like trans isomer and the globular, metastable cis isomer, units. The ionic bond consists of a coulombic attraction between illustrated in Fig. 1a. The ease with which various azobenzene two oppositely charged molecular entities.8 Hydrogen bonding derivatives can be synthesized is advantageous for controlling refers to an electrostatic attraction between a hydrogen atom the photochemistry, which is highly dependent on the sub-

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. that is covalently attached to an electronegative atom (‘‘proton stitution pattern of the azo molecules, as amply reviewed by donor’’) and another electronegative atom (‘‘proton acceptor’’).9 Bandara and Burdette14 as well as by Bleger and Hecht.15 In Analogously, a halogen bond forms when there is a net attractive addition, trans azobenzene is an effective mesogenic core, interaction between a nucleophile and a halogen atom covalently utilized for instance in designing side-chain liquid crystalline attached to an electrophilic entity.10 Hydrogen and halogen polymers.16–19 Kato et al. demonstrated already in 1996 that bonds allow for convenient control of the polymer–chromophore azobenzene photoswitching can be used for driving a nematic– interactions by proper choice of the bond donor group, thus isotropic phase transition in a hydrogen-bonded liquid crystal- providing an attractive tool for studying the effect of the line polymer–azobenzene complex.18 strength/nature of the supramolecular interaction on the photo- Upon illumination, an azobenzene-containing sample will This article is licensed under a responsive properties of the material.11,12 reach a photostationary state between the photoinduced and thermally driven reactions. Depending on the pumping wave- length and the chemical structure of the azobenzene derivative

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. in question, either trans–cis isomerization, cis–trans isomerization, Arri Priimagi earned his PhD or continuous cycling between the two isomers will predominantly degree in 2009 from the Depart- occur. Quasi-bistable photoswitching, as illustrated in the spectra ment of Applied Physics, Helsinki of Fig. 1b, and continuous cycling between the trans and cis University of Technology (presently isomers, as illustrated in the spectra of Fig. 1c, manifest them- Aalto University), Finland. After a selves rather differently in UV-vis spectra and can bring about two-year postdoc at Chemical distinct functions, both useful in their own respect. This review Resources Laboratory, Tokyo will mostly concentrate on phenomena driven by the latter, being Institute of Technology, and a 10- essential in photoinduced motions in amorphous azobenzene- month International Fellowship at containing polymers. Politecnico di Milano, he was in The general mechanism of photo-orientation is illustrated 2014 appointed to the position of a in the upper panel of Fig. 2. To a good approximation, the tenure-track assistant professor at transition dipole moment of the trans azobenzene typically Arri Priimagi the Department of Chemistry coincides with an axis connecting the midpoints of the two and Bioengineering at Tampere benzene rings,22 rendering the molecules that are perpendi- University of Technology, Finland, where he presently works as an cular to the polarization plane of the incident light inert to associate professor. In 2014 he received an Academy of Finland the incident irradiation. Linearly polarized light thus orients research Fellowship, and in 2016 was awarded an European the molecules perpendicular to the polarization plane, and the Research Council Starting Grant entitled ‘‘Tunable Photonic resulting anisotropy can be quantifiedbymeasuringatleastthree Structures via Photomechanical Actuation’’. His research interests different interdependent figures of merit : dichroic ratio, order lie in functional soft matter, with a particular emphasis in photonic parameter, and photoinduced birefringence. The first observation and light-responsive systems. of light-induced anisotropy in azobenzene-containing materials

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Journal of Materials Chemistry C Review Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Fig. 2 Schematic of photo-orientation in an azobenzene-containing film when illuminated with linearly polarized light (top) and surface-relief grating formation when illuminated with a polarization/intensity interference pattern (bottom). Adapted with permission.36 Copyright 2010, World Scientific Publishing.

A particularly fascinating consequence of photoisomerization is polymeric mass transport under interference irradiation, over This article is licensed under a a scale of several micrometers, three orders of magnitude larger Fig. 1 (a) Azobenzene photoisomerization reaction. The reaction from than the size of the photoisomerizing units. This all-optical the metastable cis to the stable trans form can occur thermally or surface patterning, often times denoted as surface-relief grating photochemically. (b) Typical example of spectral changes in a polymer–

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. (SRG) formation, is schematically illustrated in the lower panel azobenzene complex, for which the azobenzene trans and cis bands are of Fig. 2 and was first observed in 1995.34,35 It can be detected well isolated, which can give rise to bistable photoswitchable systems. The black and red curves denote the spectra in the dark and in the photosta- in situ by monitoring light diffraction from the area of the tionary state, respectively, while the intermediate ones can be used for emerging surface pattern. Information on the patterns and thus determining the thermal half-life of the cis isomer. (c) Disperse Red 1 (DR1) mass transport can also be gathered ex situ by nanomicroscopy doped in a polymer host serves as an example of an azobenzene for which techniques such as atomic force microscopy (AFM). the trans and cis bands overlap, and the same (visible) wavelength drives Despite extensive theoretical research efforts,37–40 the origin the isomerization reaction to both directions. Fig. 1a and b adapted with permission.20 Copyright 2016, American Society of Chemistry. Fig. 1c of the SRG inscription still remains a conundrum. As Ambrosio adapted with permission.21 Copyright 2010, Optical Society of America. et al. and Fabbri et al. point out, it is likely that a combination of several microscopic mechanisms is necessary to describe all of the observed phenomena.41,42 Additionally, Seki and coworkers

dates back to 1929, when Weigert published his pioneering concluded that the SRG formation mechanism in low-Tg liquid- work,23 giving rise to the term Weigert effect, for photo- crystalline polymer films, where all of the azobenzene molecules orientation. The field was reborn in the 1970s and 1980s, with were pre-driven into a cis state, is different from that in glassy publications on photo-orientation in viscous azobenzene amorphous polymer films, where the SRG formation is based solutions,24,25 mixtures of polymers and azobenzene molecules26 on continuous cycling of the azobenzenes between the trans and covalent azobenzene-functionalized polymers.27,28 Inspired and cis states.43–45 by these early developments, reviewed in 198929 and in 1993,30 Thus, it is likely that the illumination conditions and the Natansohn and Rochon successfully developed structure– type of material in question affect the mechanisms underlying property relationship rules for amorphous, covalently- different photoinduced patterning processes. Possibly more functionalized side-chain azobenzene polymers,31,32 collected than one mechanism may be in effect simultaneously and/or in their 2002 review.33 subsequently at different phases of the surface pattern formation.

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Furthermore, the photo-orientation of the azobenzene molecules alignment surfaces,64 Lee et al. on photofluidization lithography,65 perpendicular to the polarization plane of the incident light Seki et al. on recent advancements in photoalignment,66 Natansohn occurs simultaneously with photoinduced surface patterning, and Rochon on the application potential of both photo-orientation leading to birefringence gratings. Interference patterns may also and photo-induced surface patterning,67 and Priimagi and create other periodical changes in the material making the final Schevchenko on the potential photonic applications of the diffraction grating a superposition of several gratings with photo-induced surface patterns.68 different time dynamics and relative phases.46,47 1.3. Scope of this review and complementary reading 2. The extent and the stability of the Research on organic film-forming photoresponsive materials supramolecular bond goes back to at least 1970, with the 1980s and 1990s being the golden age of research on covalently functionalized photo- In this section, we will first indicate the principal methodologies responsive azobenzene-containing polymers.30,33 To learn more used to characterize the existence and extent of supramolecular about the earlier work on organic photoresponsive materials bonding, considering both direct methods and how the presence based on covalent functionalization, we refer the reader to the of bonding can be evaluated by indirect methods, particularly based reviews by Viswanathan et al.,48 Natansohn and Rochon,33 and on material property changes. Furthermore, the methodology Shibaev and coworkers.49 Oliveira et al.50 and Advincula51 have for in situ measurements of the bond stability during photo- summarized the progress on photoresponsive layer-by-layer thin isomerization is discussed. Finally, the influence of the supra- films, not covered in the present review. In the 2000s, the focus in molecular bond strength versus competitive intermolecular material design has shifted towards supramolecular analogues of interactions on the dispersion of the photoactive guest within these polymers, typically composed of photopassive polymers the passive host is discussed. functionalized non-covalently with azobenzene molecules. A review For pre-experimental estimates of the likelihood and strength of formation of a hypothesized supramolecular bond, energy

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. of these materials covering the pre-2009 period has been published by Stumpe and coworkers.52 Many more papers on this subject minimization of the complex by density functional theory (DFT) have been published since that review, with a number of structure– is often applied, even though care should be taken to involve property–function relations elucidated. Here, we review the relevant functionals and corrections. Moreover, DFT does not progress made since 2009, focusing on the material design provide information on which of the competing interactions of concepts for fabricating efficient photoresponsive materials the material system in question are the dominating ones. To through ionic, hydrogen and halogen bonding between the include all relevant interactions in one simulated system, a more host polymers and small molecules. informative platform is molecular dynamics (MD), since it is a In 2014, Faul summarized the use of ionic self-assembly for many-body type simulation by nature. Regarding experimental This article is licensed under a supramolecular materials, briefly including photoresponsive characterization, it is imperative to consider that sample pre- azobenzene complexes,53 Ube and Ikeda reviewed recent paration conditions can have significant effects on the extent developments in photomobile azobenzene-based materials,54 of the anticipated supramolecular bonding. For example, spin- coating leads to rapid solvent evaporation, which can freeze

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. and Seki reviewed mainly covalent photoresponsive liquid crystal polymers,44 complemented by a review and a highlight in non-equilibrium supramolecular structures. This may, for article by Yu.55,56 In 2013, Priimagi et al. wrote an account on instance, lead to a higher extent of hydrogen bonding and using halogen bonding in the design of functional materials57 therefore less phase separation (if this is a tendency), as com- and, in 2015, they reviewed halogen bonding in the context pared to drop-casting or dip/blade-coating, for which slower of photoresponsive azobenzene-containing materials.58 The solvent evaporation allows dry-state equilibrium structures to interplay of light and azobenzene-containing block copolymer be approached or attained. In structured systems, such as block morphologies in solution and in the solid-state was reviewed copolymers or liquid-crystalline (LC) materials, the degree of by Zhao and He in 2009.59 Very recently, in 2017, a review of development of the morphologies formed also depends on the light-triggered topographies in azopolymers including a brief sample preparation conditions. For the former, heat and solvent description of the supramolecular work in the field was written annealing are commonly used to develop or control microphase by Broer and coworkers.60 Also in 2017, the use of azobenzene separation, and the photoresponse of the system may depend on 69–72 as a building block in molecular machine architectures61 and in the degree of self-assembly of the material. On the other light-powered molecular devices62 were reviewed. Additionally, hand, annealing after photoinduced surface patterning has been in early 2018 light control of nano-objects especially focusing on shown, under certain conditions, to enhance pre-inscribed grat- 73 using noncovalent interactions to couple azobenzene surfactants ings for amphiphilic block copolymers. to these objects was reviewed, thus indicating the timeliness of employing supramolecular interactions in the design of photo- 2.1. Characterization of the supramolecular bond responsive materials.63 Demonstrating the formation and measuring the strength of For technological applications of light-responsive materials, existing supramolecular bonds experimentally depends strongly which is out of the scope of the present review, we refer the readers on the type of supramolecular bond anticipated and on the to the reviews by Ichimura on the use of azobenzenes as LC nature (amorphous vs. organized) of the final material system.

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Recently, dynamic force microscopy (DFM) has been used to correlated with supramolecular bonding. In various systems, map changes in electron-density in two-dimensional hydrogen- successful complexation generates liquid crystallinity, often not bonded assemblies of naphthalene tetracarboxylic diimide present in the pure components and which is generally char- molecules,74 and in general, the advancement of high-resolution acterized by a combination of DSC, polarized optical micro- microscopy can further enhance the capacity to directly observe scopy (POM) and X-ray techniques (WAXD and small angle supramolecular bonds. However, typical experimental methods scattering, which can also be applied to thin films in grazing used for the characterization of supramolecular bonds in bulk incidence angle configuration).81–83 samples can be divided into direct methods that explicitly probe The combination of illumination with any of the character- the characteristics/existence of the supramolecular bonds (in an ization tools mentioned above can provide valuable insights ensemble) and indirect methods that rely on the changes in into what happens to the supramolecular bond during photo- material properties induced by supramolecular bonding. For ionic isomerization, i.e. when there is photoinduced motion in the interactions, two categories must be distinguished: those resulting material. FTIR is the easiest method to combine with incident from proton transfer (often also involving some H-bonding), illumination to gain such information in situ. For example, our such as from a sulfonic acid group to pyridine, or from studies indicate that phenolic hydrogen bond and iodoacetylene- carboxylic acid to a tertiary amine, and those implicating ion based halogen bond with poly(4-vinylpyridine) (P4VP) are relatively exchange procedures between two salts. stable upon trans–cis–trans cycling.84 Such studies, assessing The most common experimental method for characterizing the photostabilities of different supramolecular bonds, should hydrogen-bonding, halogen-bonding and ionic bonding resulting be extended, for instance, to different temperatures. Yet already from proton transfer is Fourier transform infrared spectroscopy the first experiments85 highlight that supramolecular bonds are (FTIR),75 the use of which in the context of azobenzene-materials effective for kinetic energy transfer between the phototoactive has been reviewed recently.76 It provides information on which azobenzene units and the photopassive polymer. chemical groups are involved in the supramolecular bonding and 2.2. Interplay of different interactions in supramolecular

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. the IR wavenumber shifts upon complexation can be qualitatively correlated with the supramolecular interaction strength.75 material systems Additionally, quantification of the extent of supramolecular bonding In supramolecular materials design, it is essential to take a can be reached by using appropriate model compounds.77 More holistic view, since the material components interact with one onerous methods, like X-ray photoelectron spectroscopy (XPS) another not only via site-specific supramolecular interactions, and solid-state NMR can also provide information on halogen but also via non-specific dispersion and electrostatic forces. and hydrogen bonding.78–80 For ionic bonds resulting from ion Therefore, in addition to the choice of supramolecular inter- exchange, XPS and FTIR are generally not very helpful, since the action type, the choice of the chromophore itself is important, changes are often subtle. Instead, the bonding is often deduced since its shape and dipolar character will determine the inter- This article is licensed under a from the absence of the small counterions targeted for elimina- molecular interactions among the chromophores themselves. tion (to drive the desired interaction), which can be measured by Another aspect is the chromophore-to-host ratio, which can be energy dispersive X-ray spectroscopy (EDS). varied in order to tune the properties of the resulting material.

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. For any of the supramolecular bonds, additional confirmation These combined factors determine whether the chromophores are of complexation can be obtained by verifying the molar ratios of well dispersed within the polymer host, whether they self-assemble the components using NMR spectroscopy and elemental analysis. into ordered structures, or form macrophase-separated, often This is particularly useful when excess small molecule has been times crystalline, domains. introduced to obtain complete complexation and that is sub- Overall, three main possibilities can be distinguished: sequently eliminated during ion exchange and purification (often (i) chromophore–chromophore interactions dominate the by dialysis), and when drastic drying conditions are employed chromophore–host interactions, leading to macrophase separa- with less strongly bonded small molecules to eliminate residual tion of the chromophore domains; (ii) chromophore–host solvent (e.g. under prolonged vacuum and/or high temperature). interactions are strong enough to prevent macrophase separa- Furthermore, changes in various properties of the supramo- tion, but lateral interactions between the chromophores lead to lecular complexes compared to the parent components provide self-organization of the material, particularly into LC domains; an indirect measure of the success of supramolecular bonding. and (iii) chromophore–host interactions are strong enough Typically, the photoactive small molecules in their pure form and/or chromophore–chromophore interactions are weak enough are crystalline, whereas the complexes with a host polymer to maintain both supramolecular bonding and dispersion of are non-crystalline. Thus, the absence of crystallinity – easily the chromophores, resulting in a macroscopically isotropic, detected by wide-angle X-ray diffraction (WAXD) and comple- amorphous material even at nominally equimolar chromo- mented with differential scanning calorimetry (DSC) to confirm phore concentration. In practice, different systems (or prepara- the absence of/change in a melting point – is supportive of tion conditions) may lie at different points on a continuum complete complexation, whereas its presence is indicative of outlined by these three cases. (partial) phase separation of uncomplexed material. Similarly, Case (i), in which chromophore–chromophore interactions changes in the polymer glass transition, as detected by DSC, dominate over other interactions, leads to aggregated structures particularly as a function of the small molecule content, can be similar to those of the pure molecules, which are typically crystalline.

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Crystalline aggregates can, in fact, be useful for specific applica- Contemporarily, Priimagi et al. demonstrated that the aggrega- tions. For instance, Disperse Red 1 (DR1) molecules, which show tion of Disperse Red 1 (DR1) molecules can be suppressed by a large blue-shift in the absorption maximum when they are a polymer backbone with a complementary supramolecular aggregated compared to when they are dispersed, may lead to binding site for DR1, using both hydrogen bonding and proton highly stable photoinduced anisotropy,21 and such systems have transfer interactions.89 This concept was further applied to been proposed as humidity-controlled memory materials.85 On show that photoinduced birefringence and its temporal stability the other hand, H-type and J-type aggregation of azobenzenes can be enhanced in amorphous hydrogen-bonded materials as can be used for fluorescence emission enhancement.86–88 More compared to non-bonding polymer–azobenzene mixtures.90 In specifically, the Yu group found that fluorescence measurements another highlight of supramolecular photoresponsive material can be employed as an indicator of aggregate size and tightness, design in the early years of the field, Faul, Stumpe et al. showed and that the aggregates in solution state can be disassembled by that the ionic self-assembly of an azobenzene small molecule photoisomerizing the azobenzenes with UV light.87,88 However, as a (Ethyl Orange) with surfactants can create liquid-crystalline supra- rule of thumb for solid-state polymer–azobenzene complexes, molecular complexes yielding temporally stable anisotropic azobenzene aggregation, i.e. case (i), is detrimental to both the molecular orientation, with dichroic ratios of up to 50 under isomerization reaction and the photoisomerization-induced illumination with linearly polarized light.100,101 To the best of phenomena.89,90 Case (ii), which often results in LC character our knowledge, this is still the highest photoinduced dichroic above a system-specific chromophore/host ratio, has been observed ratio reported to date. Thus, supramolecular photoresponsive both for H-bonded16,91 and ionically bonded systems.81,92,93 Case materials are capable of competing with, and even surpassing, (iii) is more likely for chromophores with low dipolar character the performance of covalently functionalized materials. In this and/or a bulkier or less linear molecular structure,94 as well as chapter, we will first concentrate on the design guidelines for for materials with low chromophore/host ratio. ionic complexes, then for hydrogen-bonded, and finally for halogen-bonded complexes.

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 3.1.1. Photoinduced anisotropy in ionically bonded complexes. 3. Supramolecular polymer– Ionic complexation of methyl orange (MO), a commercially available azobenzene complexes as azobenzene with quaternized poly(4-vinylpyridine) (P4VP or PVP) photoresponsive materials through ion exchange was reported independently by the Lu102 and Bazuin93 groups. These complexes, characterized by a very high glass In the context of photoresponsive azobenzene-based materials, transition temperature (near 180 1C) and a smectic A-type structure, theinterestinadoptingsupramolecular strategies can be related gave rise to photoinduced birefringence of 0.07 under 150 mW cm2 to three main advantages. In comparison to nonbonded guest– irradiance102 and a remarkable 0.18 under 1 W cm2 irradiance.93 host mixtures, these strategies can prevent or decrease the Shortly after, Pan et al. further highlighted the potential of This article is licensed under a aggregation of the azobenzene molecules, thus enhancing the such ion-exchange complexes as photoanisotropic materials, photoresponse of the materials.89,90,95 In comparison to covalently reporting birefringence of 0.08 in complexes of poly(1-butyl- functionalized side-chain polymers, an important advantage vinylpyridinium bromide) and MO under 800 mW cm2 concerns the ease of preparation and customization of supra- 103

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. irradiance. They also demonstrated that both the maximum molecular materials, rendering them a more cost-effective value and the kinetics of photoinduced birefringence depend and less labor-intensive choice. This is especially valuable for on the incident inscription irradiance,103 extremely important extensive fundamental structure–property relationship studies. result to keep in mind when comparing the achieved photo- Finally, the dynamic character of many noncovalent bonds can orientation values by different groups done under different be taken advantage of by, for instance, non-destructive removal illumination conditions. For all these materials, the induced of the azo molecules using selective solvation.96,97 This chapter birefringence was essentially stable after turning off the inscrip- is divided into three topical areas: photoinduced anisotropy, tion beam, which can be attributed to the rigidity of the molecular

photoinduced surface patterning, and photocontrol of block structure, with consequent high Tg, possibly combined with the copolymer micro- and nanostructures, in each of which the liquid crystallinity of the materials.93,102,103 For triazine-based use of supramolecular materials design has seen significant molecular glasses, an analogous stabilization effect was attributed progress within the past years. to liquid-crystalline character.104 For the complexes of Pan et al., it was shown that birefringence inscribed at room temperature 3.1. Photoinduced anisotropy could be enhanced by heating the films, which was attributed to As early examples of photoinduced anisotropy in supramole- liquid crystalline character of the material.103 cular complexes, Medvedev et al.98 and Cui and Zhao99 showed To examine the design guidelines for ionically bonded photo- that the liquid crystallinity and photo-orientation capability of responsive materials, Bazuin and coworkers prepared the library covalently functionalized side-chain polymers can be modified by of materials pictured in Fig. 3.82 They found that flexible hydrogen bonding small molecules to the side-chain extremities. molecular units are detrimental, in proportion to the degree In both cases, the inscribed anisotropy was temporally relatively of flexibility, to the extent and stability of the photoinduced stable (up to remnant anisotropies of 80% or more), like in birefringence. Thus, the most rigid structure, MO/PVP, resulted covalent azobenzene-containing polymers reported earlier.33 in the highest photoinduced birefringence, while both this

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crystallinity and high and stable photoinduced birefringence were obtained through acid–base titration of poly(propyleneimine) codendrimers by a COOH-capped azobenzene derivative (equimolar amounts of carboxylic acid and N-terminal amine groups).105 The best materials in terms of photoinduced anisotropy, built on

a randomly hyperbranched polymer backbone of PEI-(NMe2)n, reached photoinduced birefringence of approximately 0.13 and a remarkably high order parameter of 0.67 upon irradiation with linearly polarized 488 nm light (irradiance 1 W cm2).105 As demonstrated in Fig. 4c, the inscribed birefringence continued to increase slightly after switching off the illumination, and the inscribed birefringence was reported to be stable for several weeks.105 The same group extended the materials to a third- generation poly(propylene imine) dendrimer, which, when fully complexed with the azobenzene, CAzPA (Fig. 4a), exhibited nematic liquid crystallinity in a temperature range of 47–163 1C.106,107 The LC phase type and temperature range could be tuned by substituting CAzPA by varying amounts of a photopassive comesogen (with constant overall equimolar complexation). The dendrimer complexed with an equimolar amount of CAzPA and the photopassive comesogen showed a very broad LC temperature range, from 19 to 105 1C, and a high and stable 107

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. photoinduced order parameter of 0.63. It was also noted that the inscribed birefringence, and thus the anisotropic molecular alignment, increased somewhat after switching off the writing beam.107 3.1.2. Photoinduced anisotropy in hydrogen-bonded complexes. Following the initial reports of Medvedev et al.,98 Cui and Zhao99 and Priimagi et al.89 on H-bonded complexes mentioned above, Wang et al. demonstrated photo-orientationinsupramolecularH-bonded

Fig. 3 Top: Library of the ion-exchange polymer–azobenzene complexes materials of two different architectures; (i) main-chain polymeric This article is licensed under a studied, identified by the following acronyms: (a) H/PVP, (b) Me/PVP, (c) complexes built from complementary bifunctionalized azobenzenes Hex/PVP, (d) Ch*/PVP, (e) MO/PVP, and (f) MO/PDM, where PVP and PDM and passive units,108 and (ii) side-chain polymer complexes built refer to the methylated polyelectrolyte backbones. Bottom: Photoinduced from carboxylic acid-functionalized azobenzenes and a linear P4VP birefringence cycles of (i) inscription with a linearly polarized light for 10 s, 109

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. backbone. The former material resulted in a photoinduced order (ii) relaxation (beam off) for 30 s, (iii) erasure under circular polarization for 30 s, and (iv) beam off for 10 s in spin-coated films of the complexes. The parameter of 0.42 under the relatively moderate illumination 2 108 illumination wavelength was 488 nm and intensity 320 mW cm2.Adapted of 80 mW cm of 488 nm light. The latter material system with permission.82 Copyright 2009, American Chemical Society. exhibited dimerization of the carboxylic acid groups, competing with chromophore–polymerhydrogenbonding.Eventhoughthorough characterization of the extent of chromophore–polymer hydrogen- complex and H/PVP, where the OH tail allows intermolecular bonding was not done, the onset of solubility problems (chromo- H-bonding, yielded temporal stability of more than 98% of the phore phase separation) strongly depended on the para-substitution saturated birefringence over the time period studied (Fig. 3). An of the chromophores.109 Concurrently, Priimagi et al. demonstrated additional confirmation of the benefits of such ‘‘no-spacer’’ that, when incorporating DR1 into hydrogen-bond-accepting (P4VP) structures was reached when examining the effect of the flexible or -donating (poly(4-vinyl phenol)) polymer matrices, the photo- spacer length between the polymer backbone and the azobenzene induced birefringence is significantly enhanced and stabilized unit to the photoinduced birefringence (MO/PVP vs. MO/PDM), compared to a nonbonding polystyrene matrix, particularly at higher again demonstrating the detrimental role of flexible moieties DR1 content up to studied 30 wt% of DR1 (corresponding to 0.15 on both saturated birefringence and its temporal stability in DR1 : polymer repeat unit ratio), due to reduced aggregation.90 Later, such materials.81 Accordingly, the presence of a hexyl spacer Virkki et al. exploited the same concept in all-optical poling,110 in the polymer probably contributed to the low photoinduced which, unlike axial photo-orientation, leads to polar, noncentro- birefringence and its 10–20% relaxation observed for the smectic symmetric order of the molecules, and showed that (i) polymer– A-type LC complexes of Medvedev et al.98 chromophore noncovalent interactions play a favorable role also in As another interesting strategy, demonstrated by Sanchez nonlinear optics, and (ii) axial and polar photo-orientation exhibit et al. and depicted in Fig. 4a and b, ionically bonded (proton distinct concentration dependencies, the latter being more prone to transfer) supramolecular materials exhibiting nematic liquid chromophore–chromophore intermolecular interactions.111

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hydrogen-bonded to P4VP or poly(4-vinylphenol), served as an incitement to focus on phenolic azobenzenes and their complexes with P4VP. Hydrogen bonding between 4-nitro-40-hydroxyazo-

benzene (Fig. 5a) and P4VP (denoted as P4VP(NHA)x where x indicates the phenol/pyridine molar ratio) allowed dye–polymer Creative Commons Attribution-NonCommercial 3.0 Unported Licence. This article is licensed under a Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM.

Fig. 4 (a) Preparation method of ionic proton-transfer dendritic polymer complexes with the acid-functionalized azo derivative CAzPa, and (b) chemical structures of the dendritic polymers used. (c) Photoinduced birefringence (inscription turned on at 0 s and off at 240 s) in thin films Fig. 5 (a) Hydrogen-bonded complex between poly(4-vinylpyridine) 0 of the complex of PEI-(NMe2)n and CAzPa. The birefringence was inscribed (P4VP) and 4-nitro-4 -hydroxyazobenzene (NHA), as well as polystyrene with linearly polarized 488 nm light at irradiance of 1000 mW cm2 and it (PS) and 4-nitroazobenzene (NAB) used for two nonbonding mixtures. 105 was observed to be stable for several weeks. Adapted with permission. (b) Infrared absorption spectra of P4VP(NHA)x complexes demonstrating Copyright 2008, American Chemical Society. the shift of the 993 cm1 free pyridine stretching vibration band to higher wavenumbers upon hydrogen bonding and showing close to equimolar complexation for x = 1.0. (c) Saturated photoinduced birefringence (inscription done at 375 nm at 40 mW cm2) as a function of The fact that DR1 tends to aggregate or phase separate dye concentration for the materials studied. Adapted with permission.91 at chromophore concentrations exceeding 15 mol%, when Copyright 2008, American Chemical Society.

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ratios up to equimolar to be attained without phase separation or compromising of the film quality. Direct evidence of success- ful complexation was obtained from IR absorption spectra (Fig. 5b) showing a systematic increase in the intensity ratio of the hydrogen-bonded pyridine band at 1009 cm1 relative to the uncomplexed pyridine band at 993 cm1 as a function of increasing degree of complexation. Fig. 5c illustrates that the saturated photoinduced birefringence values are systematically higher for hydrogen-bonded P4VP(NHA) complexes as com- pared to nonbonding PS(NHA) and P4VP(NAB) mixtures. Over- all, the complexes of this study resulted in significantly higher photoinduced birefringence values (0.15 for the equimolar

P4VP(NHA)1.0) compared to P4VP(DR1) complexes (0.04 for 30 wt% DR 1 in PVPh, the highest chromophore content 111 studied). P4VP(NHA)1.0 also showed an impressive temporal stability of over 98%.91 Another key advantage of phenol–pyridine hydrogen bonding is that it facilitates structure–function comparisons, due to simple synthetic routes available for phenol-capped azobenzenes, allowing for building molecular libraries where one parameter at a time can be varied. Fig. 6 presents one such library, using P4VP as the polymer matrix. In all cases, the hydrogen-bonding

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. interaction was strong enough to prevent macrophase separa- tion of the chromophores up to equimolar ratios. This allowed assessing the effect of chromophore polarity, bulkiness, and length on photo-orientation.91,94,112–115 Focusing first on molecules 1–4 (1 = NHA above), which differ only in their substitution at the tail (nonbonding) end of the azobenzene, it was shown that this change was enough to Fig. 7 (a) Photoinduced birefringence (inscription done at 457 nm at make a drastic difference in the photoresponsive behavior of 150 mW cm2) normalized by the chromophore number density, i.e., the

the complexes. Fig. 7a shows the saturation values for photo- relative birefringence contribution per azo molecule, for P4VP(2–4)x.For This article is licensed under a induced birefringence normalized by the approximate number clarity, the lowest-concentration data point (x = 0.05) is normalized to unity. density of the chromophores in the P4VP(2–4) complexes over a Inset: Writing/relaxation curves of the photoinduced birefringence for P4VP(2)0.25 (blue curve) and P4VP(2)1.00 (red curve). (b) Normalized UV-vis

absorption spectra of spin coated thin films of selected P4VP(2)x complexes. 94 Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. Adapted with permission. Copyright 2011. Royal Society of Chemistry.

large range of complexation degrees. Such data can be used as a measure of the contribution per individual chromophore to overall birefringence.31 The most notable difference between the materials is that for P4VP(2) (the same behavior was observed also for P4VP(1),91 reported earlier) an individual chromophore contributes more to the overall birefringence at complexation degrees exceeding 0.5, whereas for P4VP(3–4), the greatest contribution per chromophore occurs at very low degrees of complexation where the chromophores presumably do not interact with one another. This can be rationalized by the fact that 1 and 2, bearing polar end groups, promote LC order in the complexes at concentrations above about 50 mol%. The onset of the appearance of an LC phase with increasing chromophore concentration can also be observed as an increased blue-shift of the UV-visible spectra of the complexes (Fig. 7b), indicative of lateral interactions between the chromophores. In contrast, molecule 3, although having a comparable dipole Fig. 6 The structures of the library of phenol–pyridine hydrogen-bonded moment, has a bulkier tail, and molecule 4, having a somewhat polymer–azobenzene complexes studied in the course of our work. lower dipole moment, result in isotropic polymer films up to

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equimolar complexation.94 These results indicate that inter- P4VP(8) showed that, in order to reach significant birefringence molecular chromophore–chromophore interactions, leading to levels, it is important to use the wavelength that drives the the formation of anisotropic LC domains, enable cooperative photoisomerization reaction in both isomerization directions photo-orientation of the azobenzenes within the supramolecular (trans-to-cis and cis-to-trans), even when that wavelength polymeric complexes, thereby increasing the saturation values of (488 nm) lies far from the absorption maximum (356 nm for the birefringence through combined photo-induced orientation P4VP(7) and 354 nm for P4VP(8)).115 In general, the ability to and ‘‘dipolar dragging’’ of adjacent chromophores. record birefringence at wavelengths barely absorbed by the Similar behavior was observed by Wu and coworkers in a material may be beneficial for applications such as optical covalently bonded side-chain azopolymer bearing azopyridine units holography,118,119 where low penetration depth of conventional that were hydrogen-bonded to another azobenzene to yield supra- azobenzene-containing polymers may become a limiting factor. molecular bisazobenzene complexes.116 At an equimolar degree of One pertinent question provoked by these studies is the role complexation, photoinduced birefringence of 0.1 was reported of the strength of the hydrogen-bonding interaction. With this (inscription done at 442 nm with an intensity 317 mw cm2), in mind, we attempted to extend the library of azobenzenes to a which increased slightly after ceasing the irradiation.116 By COOH-capped and an SH-capped dye acting as a stronger and a varying the polarization and intensity of the writing beam in weaker hydrogen-bond donor, respectively. However, experi- a two-dimensional plane, four-dimensional optical memories mental problems were encountered due to the tendency of the with a storage density of 0.93 Gbit cm2 (corresponding to COOH-capped azobenzenes to self-associate and SH-capped ones approximately 20 times the information density of a typical to dimerize through S–S bonds, thus leading to crystallization and DVD) were demonstrated. The stored information could be read phase separation in the polymer matrix. A similar tendency for with a polarized optical microscope. They could also be erased COOH-functionalized azobenzenes in P4VP was observed by del by heating, and subsequently rewritten.116 Barrio et al., who showed that phase separation occurs for degrees An infrared technique called polarization-modulation FTIR of complexation greater than about 0.50 for a carboxy-capped 127 120

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. structural absorbance spectroscopy (PM-IRSAS), which can cyanoazobenzene containing a flexible alkyl spacer. decouple the orientational effects originating from different To circumvent the inherent dimerization tendency of chemical groups of the material system, was used to follow carboxylic acid-functionalized azobenzenes at higher degrees the molecular-level kinetics of photo-orientation induced by of complexation, a dendritic molecule combining three azo- linearly polarized light and its subsequent thermal relaxation in benzene moieties, dAZO, was synthesized and hydrogen-bonded the complexes of 2 and 4 with P4VP.117 This study showed that, to P4VP, as shown in Fig. 8.121 This allows a high number of active while the saturated orientation per azo of both 2 and 4 azobenzene units for a lower number of hydrogen-bonding units. decreases with increasing azo content, 2 orients more than 4 Another strategy to inhibit COOH dimerization is to modify the and only 2 also drives photo-induced orientation of the passive backbone, allowing it to accept multiple hydrogen bonds from This article is licensed under a H-bonded pyridine groups of P4VP. Furthermore, it was con- one tecton by polymerizing a methacrylic monomer containing cluded that 4 undergoes mainly angular hole burning resulting a 2,6-diacylaminopyridine unit, thus increasing the total from angular-dependent trans–cis isomerization, whereas for 2 amount of dye loading by suppressing the phase separation 122

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. there is a significant additional contribution from angular of tectonic units at degrees of complexation exceeding 0.50. redistribution. The latter, promoted by azo–azo interactions, The reached 1 : 1 (tecton : polymer repeat unit) complex, which leads to reorientation of the trans isomers of 2 and the pyridine is liquid crystalline, yielded essentially stable photoinduced groups to which they are H-bonded, as well as to greater anisotropy.122 orientational stability. These results explain the contrasting photo- induced birefringence results between P4VP(2)andP4VP(4)shown in Fig. 7 and illustrate how small differences in the azo molecule (the tail group here) can lead to different photo- orientation mechanisms. Complexes of P4VP(5–8) were found to be isotropic through- out the complexation range studied, probably due to rotating side groups and/or nonlinear shape, both hindering molecular packing. Even though isotropic, photo-orientation was observed in all of these materials, and when comparing P4VP(5)to P4VP(8), it was shown that the larger aspect ratio of 5 gave rise to a 4-fold increase in birefringence when using the same azobenzene concentration.115 Also, two conclusions of practical importance for the selection of inscription wavelength were drawn. First, the electronic structure of 6 greatly broadened the Fig. 8 Azodendron dAZO composed of a benzoic acid derivative with 3 absorption spectrum, making it possibletoinscribebirefringence alkoxy-cyano-azobenzene substituents, and structures and schematic 112 over a wide range of wavelengths, from 375 nm to 633 nm. representations of P4VP homopolymer and PS-b-P4VP block copolymer. Second, the writing wavelength comparison for P4VP(7) and Adapted with permission.121 Copyright 2014, American Chemical Society.

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3.1.3. Photoinduced anisotropy in halogen-bonded complexes. by the lower absorption) may be preferred. Halogen bonding, as The first halogen-bonded photoresponsive small-molecule complex the newcomer in this field, is very promising, given its highest was reported in 2012, for which an order parameter of 0.5 could directionality of the three bonding types under consideration. be inscribed by illumination with linearly polarized 488 nm light (intensity 300 mW cm2).123 Once again, the LC character of the 3.2. Photoinduced surface patterning material was interpreted to enhance the photo-orientation and The first demonstrations that hydrogen bonds97,108,109 and ionic its stability, and even though the reported dichroic ratio of 50 for bonds93,102,129 can be used to create materials that undergo a small-molecule ionically bonded complex100,101 was not efficient photoinduced surface patterning were published in reached, the study demonstrated that halogen bonding opens 2007 and early 2008. Zettsu et al. showed unambiguously, by a new class of photoresponsive materials. Halogen bonding has comparison with nonbonded or poorly bonded mixtures, that higher bond directionality124 compared to (single) hydrogen the supramolecular link between the photoactive units and the bonding, whereas ionic bonding is essentially nondirectional. passive polymer matrix is essential for the formation of surface- The directionality is a key element, for instance, in devising relief structures in liquid crystalline hydrogen-bonded polymer– halogen-bonded supramolecular liquid crystals with efficient azobenzene complexes.97 It thus functions similarly to covalent photoresponsive behavior.125,126 bonds, considering that, for example, Lagugne´ Labarthet et al. To further compare halogen bonding with hydrogen bond- had pointed out that covalently functionalized polymers result in ing, we employed PM-IRSAS to show that the passive pyridine surface-relief gratings that are superior in terms of modulation moiety in P4VP can rotate in concert with the photo-orienting depth as compared to those in dye-doped polymers.130 azobenzene.84 Even though the interaction strength of halogen 3.2.1. Photoinduced surface patterning in ionically bonded bonding is less than that of hydrogen bonding, it gave rise to complexes. AsoneofthefirstexamplesofefficientSRGformation more pronounced pyridine orientation upon illumination. This in ionically bonded materials, Stumpe and co-workers reported a was attributed to the much higher orientation of the halogen- thermally stable SRG with a record modulation depth of 1.65 mm, for 129

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. functionalized azo unit, which in turn was related to the higher thecomplexshowninFig.9. They also demonstrated that cross- 84 Tg of the halogen-bonded complex. The advantage of the more linked polysiloxane networks ionically complexed to azobenzenes directional nature of halogen bonding has also been pointed resulted in efficient SRG formation and consideration of the out in nonlinear optics. A simple mutation of the halogen optimal rigidity of the material (tailored by varying the degree from iodine to bromine in halogen-bonded azobenzene–P4VP of cross-linking) was discussed.131 By using conceptually complexes more than doubled second-harmonic generation similar polyelectrolyte–azobenzene complexes as in ref. 129, upon all-optical poling of the materials.128 which were spin-coated on substrates containing plasmonic 3.1.4. Summary for photoinduced anisotropy. To sum up the gold nano-antennas, the imprint of the plasmonic near field reviewed studies, some guidelines for supramolecular materials could be inscribed onto the polymer surface.132 The authors also This article is licensed under a with high photoinduced anisotropy can be drawn. First, phase noted that the polymer–azobenzene complexes were sensitive separation and crystallization should be avoided. This means to humidity due to their highly charged nature. Furthermore, overcoming the intermolecular interactions between the the Santer group showed that in the ionically-bonded

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. azobenzenes by sufficiently strong supramolecular bonds, by polyelectrolyte–azobenzene complexes, the photoinduced mass decreasing the dipole moment of the azobenzenes, or by introduc- transport direction was towards intensity maxima, whereas in a tion of steric elements hindering crystallization of the azobenzenes. linear covalently-bonded azopolymer, the direction of mass In general, liquid crystal character in the complexes, promoting transport is reversed.133 cooperative movement and intermolecular interactions of the The characteristics of efficient SRG formation in the time- photochromic molecules while avoiding crystallization, scale of minutes (at lower irradiance) were similarly observed appears to be advantageous for high photoinduced anisotropy for the MO/P4VP complex (Fig. 3), for which gratings of 360 nm with long-term stability. When comparing LC materials, redu- modulation depth, exceeding the original film thickness of cing the number of flexible units in the starting molecules leads 305 nm, could be written within 5 min with the relatively low to higher efficiency of photo-orientation, and, all in all, the irradiance of 65 mW cm2.82,93 The high photoinduced surface supramolecular strategy can greatly facilitate the preparation of patterning efficiency was attributed to the strong ionic inter-

relatively rigid ‘‘no-spacer’’ side-chain polymers. actions, the molecular rigidity, the very high Tg, and the LC The choice of bonding type depends also on what the user is nature of the complex. When comparing the MO/P4VP complex aiming for. Although the highest photoinduced anisotropy thus with the other ionically bonded materials presented in Fig. 3, far has been observed in complexes of small molecules,100,101 the polymer materials reviewed here have the advantage of easy processability and good film-forming capacity. Generally, increasing the number of photoactive azobenzene units will increase the saturation value of the photoinduced birefringence

(provided that no phase-separation of the chromophores Fig. 9 Chemical structures of the constituents, poly(diallyldimethyl- occurs); however, for some applications, lower azobenzene ammonium chloride) (PDADMAC) and methyl red (MR), of one of the content and thus lower absorption (or thicker films enabled ionically bonded polymer–azobenzene complex studied in ref. 129.

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Fig. 10 (a) Chemical structure of an ionically bonded complex of ethyl orange (EO) and a third-generation dendrimer. (b) The nominal molecular weight and experimentally determined diameters of the dendrimers, the number of binding sites in each dendrimer, and the molecular weight of nominally stoichiometric dendrimer–EO complexes. Adapted with 134

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. permission. Copyright 2014, American Chemical Society.

the same correlation as for photo-orientation was found: the more rigid the complex, the more efficient the SRG formation. From this point of view, the complex in Fig. 9 is also a very rigid system, leading to very efficient SRG formation. It should be emphasized that the polymer molecular weight in both types 82 of ionic complexes was high (200 000 for P4VP in Fig. 3 and Fig. 11 (a) Molecular structure of a hydrogen-bonded poly(vinylphenol)–

This article is licensed under a 129 ca. 500 000 for PDADMAC in Fig. 9 ). azobenzene complex, an example of an inscribed SRG and a cross-section In addition to linear polyelectrolyte backbones, dendritic of an inscribed SRG determined by AFM. (b) The first-order diffraction polyelectrolyte backbones have been used as building blocks efficiency during SRG inscription for three different molecular weights of 137 134,135 the polymer backbone. Adapted with permission. Copyright 2009,

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. for ionic dendrimer–azobenzene complexes. As opposed American Chemical Society. to linear polymer chains, dendrimer structures allow increasing the molecular weight and azobenzene content in the material while retaining the globular shape of the molecules.136 As Regarding the library of hydrogen-bonded materials pre- indicated in Fig. 10, up to very large globular supramolecular sented in Fig. 6, it was found that the lateral intermolecular complexes of generation 10 could be prepared and made to interactions leading to formation of LC domains with enhanced move in response to light.134 These molecules can be consid- photo-orientation, counteracted the formation of photoinduced ered as model systems for large biomolecules such as native surface patterns. In particular, of the monoazo compounds 2–4, proteins, which, when complexed with azobenzenes, could also the amorphous P4VP(3) complexes were found to be superior in undergo photoinduced mass transport.134 SRG inscription efficiency as compared to P4VP(2)orP4VP(4).94 3.2.2. Photoinduced surface patterning in hydrogen-bonded When comparing the bisazobenzene complex P4VP(7)inFig.6 complexes. Following the first reports of inscribing surface-relief with the analogous monoazo complex, P4VP(8), it was found that gratings in H-bonded materials,97,108,109 Priimagi et al. showed, using SRG inscription in the former is more efficient than in the latter. poly(4-vinylphenol) and a pyridine-capped azobenzene (Fig. 11a), This was attributed to the fact that photoisomerization of a that SRG formation increases systematically with increasing azo- bulkier molecule requires a larger sweep volume and thus benzene content up to nominally equimolar complexation ratio.137 induces more motion into the material.115 Moreover, similarly A similar linear tendency, though over much narrower concentration to photo-orientation, the wide absorption spectrum of P4VP(5) range, was observed for complexes of P4VP and carboxylic acid- made it possible to inscribe SRGs over a very broad spectral capped azobenzene (investigated up to 40 mol% azo content).109 It range, from 405 nm to 633 nm.112 was also found that the molecular weight of the polymer backbone For some subsequent studies on hydrogen-bonded materials, in the complexes correlates negatively with the photoinduced surface P4VP(3) was selected as the standard. This is because it remains patterning efficiency, as illustrated in Fig. 11b.137 amorphous, with the chromophore well dispersed into the

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Fig. 12 Scanning electron microscopy (SEM) images of surface patterns with spatially varying grating vector directions, created by subsequent optical inscription and erasure steps. Reprinted with permission.138 Copyright 2015, Royal Society of Chemistry. Fig. 13 Chemical structures of analogous hydrogen-bonded, proton transfer and ion exchange complexes, as well as the first-order diffraction 83 polymer, up to equimolar complexation. One recent study efficiency upon SRG inscription. Adapted with permission. Copyright 2016, American Chemical Society. concentrated in finding the lower limit of the amount of azobenzene needed to create SRG patterns. It was found that as little as 1 mol% (2.2 wt%) of 3 was enough to result in a The effect of hydrogen bonding on SRG formation efficiency grating with a modulation depth of 170 nm.113 This surprising has been compared recently with that of proton-transfer ionic result indicates that even if 9 out of 10 polymer chains bonding and of pure ionic bonding obtained by ion exchange, 83 (99 out of 100 repeat units) are statistically free of azobenzene, using the analogous complexes shown in Fig. 13. The Tg of macroscopic mass transport nevertheless occurs. This provokes the the starting polymer is low (20 1C) but increases with degree of question as to whether the hydrogen bonds are ‘‘photodynamic’’, complexation and is very high for the purely ionic complex 1 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. hopping between different polymer chains, while still strong (180 C). As shown in Fig. 13, the proton transfer complex, enough to drive mass transport, an issue that still needs to despite some crystallization of the azobenzene molecules, led to be clarified. greater SRG formation efficiency than the H-bonded complex, The same material system, P4VP(3), was used to confirm whereas, most impressively, the ion-exchange complex led to that the SRG formation efficiency depends inversely on the much greater SRG formation efficiency. This result suggests that

chain length of polymers, and thus on Tg, both with conven- ionically bonded and H-bonded complexes may have an inverse 138 tional interference patterns of circularly polarized light and dependence of SRG efficiency on molecular weight and the Tg with a nonconventional s–s interference pattern, which pre- and that the role of these parameters in SRG formation is not yet viously had resulted in very poor SRGs.139 These gratings could fully understood. This article is licensed under a also be erased by illuminating them with uniform light to 3.2.3. Photoinduced surface patterning in halogen-bonded induce inverse mass transport. Using light as an erasing tool complexes. Substituting fluorinated azobenzenes with different instead of the more commonly used heating method has the halogenatoms,I,BrandF,asshowninFig.14a,allowsfortuning 144,145

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. advantage that light has a much higher spatial and temporal the halogen bonding interaction strength with pyridine. As resolution, thus allowing more complex surface patterns to be Fig. 14b and c show, there is a direct correlation with the halogen prepared, as demonstrated in Fig. 12.138 In addition, this bond strength and SRG formation efficiency, the iodine-capped material has been used in recent explorations of photo- (resulting in relatively strong halogen bonding) azobenzene induced willow structures under uniform single beam illumi- complexed with P4VP resulting in the highest and fluorine- nation but with substantially higher incident irradiance as capped (no specific interaction with P4VP) resulting in the compared to SRG inscription.140,141 lowest SRGs. Moreover, it was shown that in the concentration Sobolewska and coworkers explored a library of different range studied (10–20 mol%), the halogen-bonded polymer– hydrogen-bonded polyimides and their capacity for photo- azobenzene complexes outperform the corresponding hydrogen- induced surface patterning.142,143 While acknowledging that bondedanaloguesinSRGformationefficiency,eventhoughthe more work is required to establish conclusive structure–property hydrogen bond interaction is significantly stronger.144,145 This design guidelines for the polyimide-based complexes, they was attributed to the higher directionality of the halogen bond clearly showed that the chemical structure of the backbone compared to hydrogen bond. Thus, both the nature and strength and whether it contains OH or COOH groups (the latter being of the supramolecular bond make a significant difference in the subject to intra- and intermolecular hydrogen bonds between SRG formation efficiency.145 Very recently it was also demon- polymer repeat units) are among the determining factors for strated that the replacement of 1 in Fig. 14a by iodoacetylene- photoinduced surface patterning efficiency. This highlights the capped nonfluorinated azobenzenes in complexes with P4VP fact that photoinduced surface patterning requires the motion of reduces the aggregation tendency. Therefore, SRGs could be the whole material system, and thus the interactions between efficiently inscribed even at high azobenzene concentrations.146 polymer chains may be as important, or in some cases even more 3.2.4. Summary for photoinduced surface patterning. To sum important, than the interactions between the polymer host and up the reviewed studies on photoinduced surface patterning in azobenzene guest. supramolecular materials, ionic bonding appears to contrast

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complexes, in turn, indicated better performance by the halogen-bonded materials (even if limited to a fairly narrow range of chromophore content).144,145 However, more studies are needed to solidify these correlations and to explain the potential mechanism differences between the complexes utilizing different noncovalent interactions. Like for the inscription of photoinduced anisotropy, the choice of material for surface structuring depends on the needs of the application in question. If thermal and chemical stability is sought for, or if high SRG formation efficiency coupled to high photo-induced anisotropy is needed, ionic complexes are the materials of choice. If the azobenzenes are to be selectively removed after, for instance, an SRG is formed, weaker hydrogen- and halogen-bonding are better adapted to such removal. Also, to our knowledge, photoinduced erasure of SRG patterns that works well for hydrogen-bonded polymer–azobenzene complexes of relatively low azo content,113 has not yet been systematically studied in ionic or halogen-bonded complexes.

3.3. Photocontrol of micro- and nanostructures through supramolecular strategy Supramolecular control has also been used for block copolymers

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. (BCPs), which generally self-organize into domains with charac- teristic length scales of 10–100 nm due to block immiscibility.13 If one block bears units that are suitable for supramolecular bonding, the small molecules comprising a complementary functionality will selectively reside in that block, thus changing the relative block volume ratio and often altering the BCP morphology. Pioneering studies along these lines were done by Ikkala, ten Brinke and coworkers, using, for example, phenol– pyridine hydrogen bonding between functionalized alkyl chains This article is licensed under a such as pentadecylphenol (PDP) and PS-b-P4VP (PS:polystyrene).147 However, none of the BCP complexes they prepared involved azobenzene guests. On the other hand, Xu and co-workers,148 Fig. 14 (a) Chemical structures of the halogen-substituted azobenzenes 149–151

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. Stamm and coworkers, and, recently Gro¨schel and used. (b) First-order diffraction efficiency and (c) resulting sinusoidal coworkers,152 have used azobenzene-containing hydrogen- modulation depth of surface-relief gratings on films of halogen-bonded P4VP–azobenzene complexes, for which the computed halogen-bonding bonded side chains in investigations involving thin-film interaction strength between ethylpyridine and the chromophores are nanopatterning, nanoparticle assembly, and templating with 5.1 kcal mol1 (chromophore 3, blue), 3.5 kcal mol1 (chromophore supramolecular BCPs, but they did not explore the potential 1 145 2,red)and0kcalmol (chromophore 1, black). Adapted with permission. photoresponsive properties of these complexes. Copyright 2015, Royal Society of Chemistry. Photoinduced control over the phase structure of BCP materials combined with supramolecularly bonded azobenzene significantly with hydrogen and halogen bonding. For ionically molecules is a highly interesting phenomenon, which has bonded complexes exhibiting an LC phase, the reduction only recently begun to be explored. One of the first studies of flexible moieties in the structures correlated with better was reported by del Barrio et al., who showed that, when SRG formation efficiency. In contrast, for hydrogen-bonded photoinduced anisotropy is inscribed in self-assembled BCP materials, introduction of bulky functional groups appeared structures composed of PS-b-P4VP and an azobenzene hydrogen- necessary to inhibit LC formation and allow for efficient SRG bonded to the minority P4VP block, the extent of photo- formation. These differences, combined with different molecular orientation is dependent on the block copolymer morphology, weight dependencies in ionically bonded and hydrogen-bonded and lamellar structure resulted in higher photo-orientation than complexes, may even suggest different dominating SRG for- spherical structure.120 To further highlight the effect of self- mation mechanisms in different types of materials. One of the assembly on anisotropy, both the photoinduced birefringence first attempts to compare SRG formation in ionically bonded and order parameter in self-assembled structures were measured materials and in hydrogen-bonded ones concluded that the tobesmallerthaninananalogousamorphousP4VP–azobenzene former seemed more efficient in the conditions investigated.83 complex, thus suggesting that a rigid polymer environment A comparison between hydrogen-bonded and halogen-bonded influences the efficiency of photoinduced processes.120

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Inscribing SRGs into BCPs creates its own challenges and opportunities, since the azobenzene-containing phase is not necessarily continuous. In 2006, the photoinduced alignment of a self-assembled cylindrical morphology upon SRG inscrip- tion in a covalently-synthesized azobenzene-containing diblock copolymers was observed,153 and the fundamental understand- ing of these alignment processes has recently been reviewed by Seki.154 Applying UV intensity interference patterns to diblock copolymer brushes covalently attached to flat solid substrates and ionically complexed with azobenzene, it was found that the local variation in azobenzene photoisomerization leads to the formation of SRG patterns accompanied by optomechanical scission.155 One interesting SRG-forming azobenzene-containing BCP is a polyelectrolyte block copolymer composed of two different poly(ferrocenylsilane) blocks, one of which contains Fig. 15 Topographical AFM images (1 1 mm2) of thin diblock copolymer azobenzene.69 This material exhibited a cylindrical microphase- films of PS–P4VP dip-coated in the dark, PS–P4VP(BHAB), dip-coated in separated structure, with the azobenzene-containing block the dark, and PS–P4VP(BHAB) dip-coated under 365 nm LED irradiation, all at a withdrawal rate of 0.5 mm min1 from THF solutions. Schematic forming the continuous phase. The SRG modulation depth diagram of the dip-coater setup and UV-visible spectra of a PS–P4VP(BHAB) was shown to be enhanced by solvent annealing, and further- solution (magenta) and spin-coated thin film (black), before (solid lines) more, the structure could be swollen by oxidizing the ferrocene and directly after (dashed lines) irradiation. Adapted with permission.156 units, resulting in a significant increase in the SRG modulation Copyright 2015, American Chemical Society. depth. Finally, the authors showed that the organic parts of these

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. polymers can be removed via plasma treatment, leaving behind an etch-resistant ceramic grating.69 the dark. This increase, along with the greater volume asso- Recently, Vapaavuori et al. demonstrated that the BCP ciated with the cis isomer, rationalizes the observed photoin- morphology of a thin film containing a block-selective hydrogen- duced morphology change. bonding azobenzene can be manipulated with light.156 As expla- These results show that light can be used to command the natory background, some of the authors had previously shown nanostructured morphology of dip-coated thin films of supra- that when the films are prepared using the dip-coating technique molecular BCPs. An additional advantage of the supramolecu- (i.e. where a substrate is pulled out of the polymer solution at a lar strategy was also demonstrated; namely, that the BHAB given rate), the hydrogen-bonding small molecule content in molecules can be removed from the films after dip-coating by This article is licensed under a the film is controlled by solution azo/polymer repeat unit ratio, selective solvation without changing the morphology. This dip-coating withdrawal speed, and the solvent used.157,158 The manipulation renders the films optically transparent and is latter two are especially critical at very slow dip-coating not available for conventional covalently bonded azobenzene

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. speeds in the so-called capillarity region, and can cause film side-chain polymers. morphology changes as a function of dip-coating rate. Vapaa- Another way of combining supramolecular material design vuori et al. took advantage of this dependence by comparing the and top-down photocontrol of the 3D structures was demon- film patterns obtained by dip-coating, under illumination strated by Yu et al., who succeeded in preparing light-deformable and without illumination, from a THF solution of PS-b-P4VP liquid crystalline microparticles via a supramolecular strategy, and 4-butyl-40-hydroxyazophenol (BHAB) in a 0.25 molar ratio notably by employing an azopyridine-containing polymer relative to the pyridine repeat units.156 Normally, dip-coating of as a hydrogen-bonding acceptor and a homologous series of such supramolecular solutions without illumination at slow dicarboxylic acids as hydrogen-bond donors that also act as dip-coating rates gives rise to films whose small-molecule supramolecular crosslinkers.159 As illustrated in Fig. 16, applica-

content is much lower than in solution but increases with tion of 360 nm UV-light at a temperature of Tg +101Cresultsin increasing dip-coating rate, as observed also with BHAB. This both a photoinduced LC phase transition and deformation of led to a change in film morphology from spherical to cylindrical the microparticle shape. Later, the Yu group demonstrated that at a dip-coating rate of around 1.0 mm min1 for the system an analogous material design combined with slow evaporation studied. When this same system is dip-coated while being from a mixture of good and poor solvents produces walnut-like simultaneously illuminated by a 365 nm LED, the cylindrical wrinkled photoresponsive microparticles.160 In addition, they morphology was obtained already at 0.5 mm min1. A compar- showed that complexes of the same azopyridine-containing ison of the surface morphologies of the films obtained at this polymer and poly(acrylic acid) results in the formation of ultra- dip-coating rate, showing dots (indicative of spheres) and thin Janus films exhibiting wetting properties that depend on stripes (indicative of horizontal cylinders) for dip-coating with the film side.161 the light off and on, respectively, is shown in Fig. 15. It was also Block copolymers in solution bring an additional twist to self- noted that the BHAB content in the dip-coated film is increased assembly phenomena, since the interactions with the solvent by dip-coating under illumination compared to dip-coating in molecules may allow formation of micellar structures or gels.

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Fig. 17 (a) Chemical structures of the employed hydrogen-bonded block copolymer complexes. (b) Schematic model of hierarchical structure formation by self-assembly of BCP cylinders (smaller scale cylinders) and

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. laser ablation (larger scale cylinders). (c) Colors resulting from hierarchical photonic structures of these samples fabricated on a silicon wafer (left) and on a glass slide (right). Reprinted with permission.163 Copyright 2012, Royal Society of Chemistry.

structure was created. The smaller length scale of the BCP self- assembly could be tuned by changing the chemical composi- tion, and the larger length scale by changing the polarization of the interference beams. Unlike the previous example, this

This article is licensed under a patterning method is not based on the photoresponse of the Fig. 16 Optical micrographs of supramolecular microparticles before (a) and azobenzene molecules, and is thus not limited to azobenzene- after (b) illumination at 360 nm. The inset illustrates through polarized optical microscopy that a photoinduced liquid crystalline–isotropic phase transition containing materials. occurs simultaneously with shape deformation of the microparticles. Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. Reprinted with permission.159 Copyright 2011, American Chemical Society. 4. Concluding remarks and outlook

In this review, we have discussed the developments in the field By synthesizing copolymers of thermoresponsive pNIPAM and of supramolecular photoresponsive polymer materials during carboxylic-acid-functionalized azobenzene-containing mono- the last decade. During this time period, the most commonly mers, the Yu group were able to discover a region, in which used noncovalent interactions to efficiently integrate photo- fluorescence from aggregated azobenzene in the micellar struc- responsive small azobenzene molecules into polymers or other tures can be enhanced by photoisomerization with UV-light.88 large amorphous macromolecules have been ionic, hydrogen Moreover, the fluorescence of the aggregates responds to and halogen bonding. In addition to strength, these three changes in temperature and pH, thus showing how clever supramolecular interactions differ in nature, which is most clearly molecular design and interplay with solvent can lead to truly viewed in their different methods of formation and directionalities. multiresponsive structures. Additionally, by applying the homo- Ionic interactions have no preferred directionality, whereas polymer of carboxylic acid functionalized azopolymer in DMSO, halogen bonding is highly directional due to the narrow con- UV-irradiation was used to trigger a gel–sol phase transition.162 finement of the electropositive bond-donating area on the Finally, hierarchical structures involving block copolymer self- polarizable halogen atom. assembly and laser ablation were demonstrated, as depicted in From the point of view of preparation, hydrogen- and halogen- Fig. 17b.163 By employing solvent annealing of PS–P(2/4)VP con- bonded materials, as well as ionically bonded ones involving taining hydrogen-bonded azobenzene molecules, as shown in proton transfer, are more versatile, requiring simple mixing of Fig. 17a, vertically oriented cylindrical structures were obtained. the components in a suitable solvent followed by solvent removal. When these were then exposed to a single laser shot interference In contrast, the preparation of ionically bonded materials pattern with a 35 ns laser pulse, a larger length-scale cylindrical obtained by ion exchange is often a multi-step procedure,

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sometimes requiring prior polymer modification (such as moieties in complexes) is an important characteristic for quaternization) and generally requiring the elimination of the improved optical response. Although this factor has not been original counterions to drive the desired ionic bond. Ionic directly compared in hydrogen- and halogen-bonded com- interactions also typically reduce the choice of appropriate sol- plexes, we note that majority of the structures of these types vents compared to hydrogen and halogen bonding. This also reported in the literature are relatively rigid ‘‘no-spacer’’ struc- means that establishing a series with varying degrees of com- tures. To further assess this structure–property relation, flexible plexation for fundamental structure–property studies is more moieties should ideally be incorporated into the host material, straightforward with hydrogen- and halogen-bonded materials. since adding long alkyl chains to azobenzene will drastically Additionally, many device applications demonstrated to date in alter its photochemical characteristics and thus not serve for covalent systems that benefit from photoresponsive azopolymers, the intended comparison. The success of no-spacer structures such as photonic components,67,164 organic lasers165,166 and has been enabled by supramolecular design, since the prepara- conductivity switching,167–169 could benefit from employing the tion of such rigid structures through covalent polymer synthesis supramolecular materials for optimizing, e.g., the azobenzene would be more complicated. content for the described application. As another guideline, direct comparisons between different Ionic bonding, on the other hand, is generally more stable noncovalent interactions suggest that the SRG formation effi- thermally and temporally, whereas hydrogen- and halogen- ciency of ionically-bonded complexes surpasses that of bonded small molecules are more susceptible to partial loss hydrogen-bonded complexes, and halogen-bonded complexes at higher temperatures or under vacuum drying. The latter can surpasses the hydrogen-bonded ones despite the latter being a also be easily removed from the final material if desired by somewhat stronger bond. However, different bonding types selective solvation. The various advantages and limitations, as also lead to different material properties, which may account well as the most commonly used characterization methods for for or contribute to the optical performance. One important

validating the formation of the supramolecular complexes and property is the Tg of the resulting system, and, for the compar-

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. some specific properties, are collected in Table 1. We also isons made, the Tg of the ionic complexes is much higher than

emphasize that the supramolecular strategy allows the mixing that of the hydrogen-bonded complexes. Similarly, the Tg of the of different chromophores, including more than one type of halogen-bonded complex was higher than that of the analogous supramolecular bond, into the same complex, in case a larger hydrogen-bonded complex. In these comparisons, the polymer spectral response or combinations of different functions or chain length is constant (since the same polymer is used), but properties is sought. the question of how the molecular weight, including in a range

Even though the picture of structure–property–function where the Tg is essentially constant, might affect optical per- relations for efficient supramolecular azobenzene-containing formances is also of interest in determining the efficiency of the polymers is still far from complete, progress has been made in different supramolecular bond types. This article is licensed under a the last decade and a number of guidelines can be highlighted. The best performing photoinduced anisotropy materials In the core of these guidelines is the choice of type and strength generally have liquid-crystalline character, which can promote of the noncovalent bond (or bond mixture) for optimizing the cooperative molecular motion and thereby possibly drive more

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. desired optical and other properties of the material system. efficient photo-orientation and photo-induced surface patterning. One guideline, highlighted in a study of ionically bonded Although it is hard to imagine a perfect comparison between complexes, is that rigidity (i.e. lack of flexible molecular an LC and a non-LC material, since it involves modifying the

Table 1 Commonly used characterization methods as well as the advantages and limitations for polymer–azobenzene complexes employing ionic, hydrogen and halogen bonding

Common characterization Interaction type methods Advantages Limitations Ionic Successful elimination of counter ions by High photoinduced anisotropy and efficient Must eliminate original counterions, (ion exchange) EDS. Verification of stoichiometry by SRG formation achieved in same material. adding to the preparation steps. Tend elemental analysis, NMR. Phase separation not usually an issue up to to be humidity-sensitive. Changes in material system by DSC, stoichiometric chromophore content. Some POM, XRD, UV-vis. materials are water-soluble. Hydrogen Easily observed by FTIR. Verification of Easy preparation (simple mixing of the Molecular aggregation/phase bonding stoichiometry by elemental analysis, constituents at desired ratio). Very low separation can be an issue at higher NMR. chromophore content easy to achieve and chromophore content. Azobenzene Changes in material system by DSC, surprisingly efficient in SRG formation. Easy content can be reduced by excessive POM, XRD, UV-vis. to eliminate the azo molecules by selective heating/vacuum drying. solvation. Halogen Easily observed by FTIR and XPS. Ver- Easy preparation (simple mixing of the Molecular aggregation/phase bonding ification of stoichiometry by elemental constituents at desired ratio). Allows separation can be an issue at higher analysis, NMR. addressing the effect of interaction strength chromophore content. Azobenzene Changes in material system by DSC, by changing the halogen substitution. content can be reduced by excessive POM, XRD, UV-vis. heating/vacuum drying.

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chemical structure, maybe the closest current example, enabled by designs can be regarded as model systems for more complicated supramolecular design, is a gradual variation of the azobenzene host molecule architectures. One step at a time, we believe that content over a critical range where the structure of the material the complexity of these materials will be increased. Many cross- adopts LC phase (analogously to lyotropic liquid crystals). Studies disciplinary fields would benefit from a similar kind of simple done with hydrogen-bonded complexes unanimously point in the and systematic materials design approach for establishing direction that the introduction of an LC phase by adding more design guidelines for desired applications. Given the facility azobenzenes not only increases the overall photo-orientation and and versatility of available supramolecular approaches, there is its temporal stability, but also the contribution per chromophore room for gathering additional fundamental understanding and in the system to the total orientation. However, the picture drawn then optimizing the materials for a wealth of applications. for the relationship of liquid crystallinity and SRG formation is not yet settled, since isotropic hydrogen-bonded materials sometimes outperform the liquid-crystalline ones, and both highly efficient Conflicts of interest liquid-crystalline and non-liquid-crystalline materials have been reported. There are no conflicts to declare. Whether the strength of the supramolecular interaction needs to be augmented or decreased for better performance is an intriguing question. Overall, the ionic bonds studied thus Acknowledgements far are at least an order of magnitude stronger than the studied JV acknowledges funding from the Academy of Finland and a hydrogen and halogen bonds. Taking into account the general Banting Postdoctoral Fellowship (Canada). AP is thankful for good performance of the reviewed ionic complexes, as well as the Academy of Finland (decision numbers 277091 and 312628) the direct correlation of increasing halogen-bond strength and and the Emil Aaltonen Foundation for financial support. GB increased efficiency in photoresponse, the field would benefit acknowledges support from the Natural Sciences and Engineering

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. from studies designing stronger hydrogen and halogen bonds Research Council (NSERC) of Canada and the Fonds de Recherche than have been studied to date. However, using less strong du Que´bec - Nature et Technologies (FRQNT). interactions might be beneficial, when preparing complexes of very low azobenzene contents, since the dynamic character of hydrogen and halogen bonding may allow hopping of the Notes and references azobenzenes between different polymer repeat units, which is hypothesized to increase surface-relief formation efficiency. To 1 J.-M. Lehn, Angew. Chem., 1988, 100, 91–116. study this hypothesis, libraries of gradually varying azobenzene 2 J.-M. Lehn, Acc. Chem. Res., 1978, 11, 49–57. contents of ionically-bonded complexes should be prepared 3 J.-M. Lehn, Pure Appl. Chem., 1978, 50, 871–892. This article is licensed under a and compared to existing hydrogen- and halogen-bonded 4 J.-M. Lehn, Supramolecular Chemistry: Concepts and Per- materials. This is of interest, since using very low azobenzene spectives, Wiley-VCH (Verlag), Weinheim, Germany, 1995. contents would allow using thicker films for photo-orientation 5 J. Steed and J. Atwood, Supramolecular Chemistry, John

Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. or (semi)-transparent films. Wiley & Sons Ltd., 2000. Completing the understanding of parameters that make or 6 C.F.J.FaulandM.Antonietti,Adv. Mater., 2003, 15, 673–683. break the performance of a material system would definitely be 7 T. Steiner, Angew. Chem., Int. Ed., 2002, 41, 48–76. easier, if the efforts towards that goal would be less scattered. 8M.Nicˇ,J.Jira´t, B. Kosˇata, A. Jenkins and A. McNaught, IUPAC. When revisiting the reviewed literature, the reader is suggested to Compendium of Chemical Terminology, 2nd ed. (the ‘‘Gold keep in mind that the photoresponse of a material is influenced Book’’), Blackwell Scientific Publications, Oxford, 1997. by various experimental parameters such as the film preparation, 9 E. Arunan, G. R. Desiraju, R. A. Klein, J. Sadlej, S. Scheiner, ambient humidity, laser wavelength, and intensity. Thus a reliable I. Alkorta, D. C. Clary, R. H. Crabtree, J. J. Dannenberg, and straightforward comparison between experimental results for P. Hobza, H. G. Kjaergaard, A. C. Legon, B. Mennucci and different materials, even for the same material made by different D. J. Nesbitt, Pure Appl. Chem., 2011, 83, 1637–1641. groups or at different times of the year, is rarely possible. 10 G. R. Desiraju, P. S. Ho, L. Kloo, A. C. Legon, R. Marquardt, Setting benchmarks of the performance needed for applica- P. Metrangolo, P. Politzer, G. Resnati and K. Rissanen, Pure tions benefiting from photo-orientation are also required to Appl. Chem., 2013, 85, 1711–1713. facilitate comparisons between the different materials. 11 G. Cavallo, P. Metrangolo, R. Milani, T. Pilati, A. Priimagi, During the last 10 years, the materials science community G. Resnati and G. Terraneo, Chem. Rev., 2016, 116, has been able to demonstrate that supramolecular polymer– 2478–2601. azobenzene complexes can reach and even surpass the perfor- 12 P. A. Kollman and L. C. Allen, Chem. Rev.,1972,72,283–303. mance of conventional covalent side-chain azopolymers. A question 13 F. S. Bates and G. H. Fredrickson, Phys. Today, 1999, 52, 32. of critical importance is what is needed to bring this research to the 14 H. M. Bandara and S. C. Burdette, Chem. Soc. Rev., 2012, next level, whether it means taking one step towards applica- 41, 1809–1825. tions or a design that would enable a breakthrough in materials 15 D. Bleger and S. Hecht, Angew. Chem., Int. Ed., 2015, 54, performance? Many of the reviewed relatively simple material 11338–11349.

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16 J. de Wit, G. A. van Ekenstein, E. Polushkin, K. Kvashnina, 44 T. Seki, Macromol. Rapid Commun., 2014, 35, 271–290. W. Bras, O. Ikkala and G. ten Brinke, Macromolecules, 2008, 45 N. Zettsu, T. Ogasawara, R. Arakawa, S. Nagano, T. Ubukata 41, 4200–4204. and T. Seki, Macromolecules, 2007, 40, 4607–4613. 17 X. Sallenave and C. G. Bazuin, Macromolecules, 2007, 40, 46 N. C. R. Holme, L. Nikolova, P. S. Ramanujam and 5326–5336. S. Hvilsted, Appl. Phys. Lett., 1997, 70, 1518–1520. 18 T. Kato, N. Hirota, A. Fujishima and J. M. J. Frechet, 47 A. Sobolewska and A. Miniewicz, J. Phys. Chem. B, 2007, J. Polym. Sci., Part A: Polym. Chem., 1996, 34, 57–62. 111, 1536–1544. 19 S. Ujiie and K. Iimura, Macromolecules, 1992, 25, 3174–3178. 48 N. K. Viswanathan, D. Y. Kim, S. Bian, J. Williams, W. Liu, 20 M. Poutanen, O. Ikkala and A. Priimagi, Macromolecules, L. Li, L. Samuelson, J. Kumar and S. K. Tripathy, J. Mater. 2016, 49, 4095–4101. Chem., 1999, 9, 1941–1955. 21 A. Priimagi, A. Shevchenko, M. Kaivola, F. J. Rodriguez, 49 V. Shibaev, A. Bobrovsky and N. Boiko, Prog. Polym. Sci., M. Kauranen and P. Rochon, Opt. Lett., 2010, 35, 1813–1815. 2003, 28, 729–836. 22 M. Utecht, T. Klamroth and P. Saalfrank, Phys. Chem. 50 O. N. Oliveira, Jr., D. S. Dos Santos, Jr., D. T. Balogh, Chem. Phys., 2011, 13, 21608–21614. V. Zucolotto and C. R. Mendonca, Adv. Colloid Interface 23 F. Weigert, Z. Phys. Chem., 1929, 3, 377–388. Sci., 2005, 116, 179–192. 24 A. M. Makushenko, B. S. Neporent and O. V. Stolbova, Opt. 51 R. C. Advincula, in Handbook of Polyelectrolytes and Their Spectrosc., 1971, 31, 295–299. Applications, ed. S. K. Tripathy, J. Kumar and H. S. Nalwa, 25 A. M. Makushenko, B. S. Neporent and O. V. Stolbova, Opt. American Scientific Publishers, Stevenson Ranch, CA, Spectrosc., 1971, 31, 741–749. 2002, pp. 65–97. 26 T. Todorov, L. Nikolova and N. Tomova, Appl. Opt., 1984, 52 J. Stumpe, O. Kulikovska, L. M. Goldenberg and 23, 4588–4591. Y. Zakrevskyy, in Smart Light-Responsive Materials, ed. 27 K. Anderle, R. Birenheide, M. Eich and J. H. Wendorff, Y. Zhao and T. Ikeda, John Wiley & Sons Ltd., Hoboken,

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Makromol. Chem., Rapid Commun., 1989, 10, 477–483. NJ, USA, 2009, pp. 47–94. 28 M. Eich, J. H. Wendorff, B. Reck and H. Ringsdorf, Makro- 53 C. F. Faul, Acc. Chem. Res., 2014, 47, 3428–3438. mol. Chem., Rapid Commun., 1987, 8, 59–63. 54 T. Ube and T. Ikeda, Angew. Chem., Int. Ed., 2014, 53, 29 G. S. Kumar and D. C. Neckers, Chem. Rev., 1989, 89, 10290–10299. 1915–1925. 55 H. Yu, J. Mater. Chem. C, 2014, 2, 3047–3054. 30 S. Xie, A. Natansohn and P. Rochon, Chem. Mater., 1993, 5, 56 H. Yu, Prog. Polym. Sci., 2014, 39, 781–815. 403–411. 57 A. Priimagi, G. Cavallo, P. Metrangolo and G. Resnati, Acc. 31 D. Brown, A. Natansohn and P. Rochon, Macromolecules, Chem. Res., 2013, 46, 2686–2695. 1995, 28, 6116–6123. 58 M. Saccone, G. Cavallo, P. Metrangolo, G. Resnati and

This article is licensed under a 32 A. Natansohn, P. Rochon, J. Gosselin and S. Xie, Macro- A. Priimagi, Top. Curr. Chem., 2015, 359, 147–166. molecules, 1992, 25, 2268–2273. 59 Y. Zhao and J. He, Soft Matter, 2009, 5, 2686–2693. 33 A. Natansohn and P. Rochon, Chem. Rev., 2002, 102, 60 M. Hendrikx, A. Schenning, M. Debije and D. Broer, 4139–4176. Crystals, 2017, 7, 231. Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. 34 D. Y. Kim, S. K. Tripathy, L. Li and J. Kumar, Appl. Phys. 61 M. Baroncini and G. Bergamini, Chem. Rec., 2017, 17, 700–712. Lett., 1995, 66, 1166–1168. 62 O. S. Bushuyev, M. Aizawa, A. Shishido and C. J. Barrett, 35 P. Rochon, E. Batalla and A. Natansohn, Appl. Phys. Lett., Macromol. Rapid Commun., 2018, 39, 1700253. 1995, 66, 136–138. 63 S. Santer, J. Phys. D: Appl. Phys., 2018, 51, 013002. 36 A. Priimagi, M. Kaivola, M. Virkki, F. J. Rodrı´guez and 64 K. Ichimura, in Polymers as Electrooptical and Photooptical M. Kauranen, J. Nonlinear Opt. Phys. Mater.,2010,19,57–73. Active Media, ed. V. P. Shibaev, Springer, Berlin, Germany, 37 M. Saphiannikova, W. Toshchevikov and J. Ilnytskyi, 1996, pp. 138–172. Nonlinear Opt., Quantum Opt., 2010, 41, 27–57. 65 S. Lee, H. S. Kang and J. K. Park, Adv. Mater., 2012, 24, 38 M. L. Juan, J. Plain, R. Bachelot, P. Royer, S. K. Gray and 2069–2103. G. P. Wiederrecht, ACS Nano, 2009, 3, 1573–1579. 66 T. Seki, K. Fukuhara, M. Sano, M. Hara and S. Nagano, 39 C. F. P. Lefin and J.-M. Nunzi, Pure Appl. Opt., 1998, 7, Mol. Cryst. Liq. Cryst., 2015, 614, 118–123. 71–82. 67 A. Natansohn and P. Rochon, Adv. Mater., 1999, 11, 1387–1391. 40 S. Ciobotarescu, N. Hurduc and V. Teboul, Phys. Chem. 68 A. Priimagi and A. Shevchenko, J. Polym. Sci., Part B: Polym. Chem. Phys., 2016, 18, 14654–14661. Phys., 2014, 52, 163–182. 41 A. Ambrosio, L. Marrucci, F. Borbone, A. Roviello and 69 R. Ahmed, A. Priimagi, C. F. J. Faul and I. Manners, Adv. P. Maddalena, Nat. Commun., 2012, 3, 989. Mater., 2012, 24, 926–931. 42 F. Fabbri, Y. Lassailly, S. Monaco, K. Lahlil, J. P. Boilot and 70 C. Frenz, A. Fuchs, H.-W. Schmidt, U. Theissen and J. Peretti, Phys. Rev. B: Condens. Matter Mater. Phys., 2012, D. Haarer, Macromol. Chem. Phys., 2004, 205, 1246–1258. 86, 115440. 71 S.-C. Tsao and C.-T. Lo, RSC Adv., 2014, 4, 23585–23594. 43 J. Isayama, S. Nagano and T. Seki, Macromolecules, 2010, 72 K. Moriya, T. Seki, M. Nakagawa, G. Mao and C. K. Ober, 43, 4105–4112. Macromol. Rapid Commun., 2000, 21, 1309–1312.

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73 H. Yu, K. Okano, A. Shishido, T. Ikeda, K. Kamata, 99 L. Cui and Y. Zhao, Chem. Mater., 2004, 16, 2076–2082. M. Komura and T. Iyoda, Adv. Mater., 2005, 17, 2184–2188. 100 Y. Zakrevskyy, J. Stumpe and C. F. J. Faul, Adv. Mater., 74 A. M. Sweetman, S. P. Jarvis, H. Sang, I. Lekkas, P. Rahe, 2006, 18, 2133–2136. Y. Wang, J. Wang, N. R. Champness, L. Kantorovich and 101 Y. Zakrevskyy, J. Stumpe, B. Smarsly and C. F. Faul, Phys. P. Moriarty, Nat. Commun., 2014, 5, 3931. Rev. E: Stat., Nonlinear, Soft Matter Phys., 2007, 75, 031703. 75 W. O. George and R. Lewis, Hydrogen Bonding. Handbook of 102S.Xiao,X.LuandQ.Lu,Macromolecules, 2007, 40, 7944–7950. Vibrational Spectroscopy, John Wiley & Sons, Ltd, 2006. 103 X. Pan, S. Xiao, C. Wang, P. Cai, X. Lu and Q. Lu, Opt. 76 J. Vapaavuori, C. G. Bazuin and C. Pellerin, Macromol. Commun., 2009, 282, 763–768. Rapid Commun., 2018, 39, 1700430. 104 K. Kreger, P. Wolfer, H. Audorff, L. Kador, N. Stingelin- 77 X. Wang, C. G. Bazuin and C. Pellerin, Vib. Spectrosc., 2014, Stutzmann, P. Smith and H. W. Schmidt, J. Am. Chem. Soc., 71, 18–23. 2010, 132, 509–516. 78 F. Wang, N. Ma, Q. Chen, W. Wang and L. Wang, Langmuir, 105 M. Marcos, R. Alcala´, J. Barbera´, P. Romero, C. Sa´nchez 2007, 23, 9540–9542. and J. L. Serrano, Chem. Mater., 2008, 20, 5209–5217. 79 J. Xu, X. Liu, J. K.-P. Ng, T. Lin and C. He, J. Mater. Chem., 106 S. Herna´ndez-Ainsa, R. Alcala´, J. Barbera´, M. Marcos, 2006, 16, 3540. C. Sa´nchez and J. L. Serrano, Eur. Polym. J., 2011, 47, 80 B. Fortier-McGill, V. Toader and L. Reven, Macromolecules, 311–318. 2012, 45, 6015–6026. 107 S. Herna´ndez-Ainsa, R. Alcala´, J. n. Barbera´, M. Marcos, 81 Q. Zhang and C. G. Bazuin, Macromolecules, 2009, 42, C. Sa´nchez and J. L. Serrano, Macromolecules, 2010, 43, 4775–4786. 2660–2663. 82 Q. Zhang, X. Wang, C. J. Barrett and C. G. Bazuin, Chem. 108 J. Gao, Y. He, H. Xu, B. Song, X. Zhang, Z. Wang and Mater., 2009, 21, 3216–3227. X. Wang, Chem. Mater., 2007, 19, 14–17. 83 X. Wang, J. Vapaavuori, X. Wang, R. G. Sabat, C. Pellerin 109 J. Gao, Y. He, F. Liu, X. Zhang, Z. Wang and X. Wang,

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. and C. G. Bazuin, Macromolecules, 2016, 49, 4923–4934. Chem. Mater., 2007, 19, 3877–3881. 84 J. Vapaavuori, I. T. S. Heikkinen, V. Dichiarante, 110 C. Fiorini, F. Charra, J.-M. Nunzi and P. Raimond, J. Opt. G. Resnati, P. Metrangolo, R. G. Sabat, C. G. Bazuin, Soc. Am. B, 1997, 14, 1984. A. Priimagi and C. Pellerin, Macromolecules, 2015, 48, 111 M. Virkki, M. Kauranen and A. Priimagi, Appl. Phys. Lett., 7535–7542. 2011, 99, 183309. 85 S. Schoelch, J. Vapaavuori, F. G. Rollet and C. J. Barrett, 112 J. E. Koskela, J. Vapaavuori, J. Hautala, A. Priimagi, Macromol. Rapid Commun., 2017, 38, 1600582. C. F. J. Faul, M. Kaivola and R. H. A. Ras, J. Phys. Chem. 86 L. Shi, X. Ran, Y. Li, Q. Li, W. Qiu and L. Guo, RSC Adv., C, 2012, 116, 2363–2370. 2015, 5, 38283–38289. 113 J. E. Koskela, J. Vapaavuori, R. H. A. Ras and A. Priimagi,

This article is licensed under a 87 H. Ren, D. Chen, Y. Shi, H. Yu and Z. Fu, Polym. Chem., ACS Macro Lett., 2014, 3, 1196–1200. 2015, 6, 270–277. 114 J. Vapaavuori, A. Priimagi and M. Kaivola, J. Mater. Chem., 88 H. Ren, D. Chen, Y. Shi, H. Yu and Z. Fu, Polymer, 2016, 97, 2010, 20, 5260–5264. 533–542. 115J.Vapaavuori,A.Goulet-Hanssens,I.T.S.Heikkinen, Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. 89 A. Priimagi, S. Cattaneo, R. H. A. Ras, S. Valkama, O. Ikkala C. J. Barrett and A. Priimagi, Chem. Mater., 2014, 26, 5089–5096. and M. Kauranen, Chem. Mater., 2005, 17, 5798–5802. 116 S. Wu, S. Duan, Z. Lei, W. Su, Z. Zhang, K. Wang and 90 A. Priimagi, M. Kaivola, F. J. Rodriguez and M. Kauranen, Q. Zhang, J. Mater. Chem., 2010, 20, 5202–5209. Appl. Phys. Lett., 2007, 90, 121103. 117 X. Wang, J. Vapaavuori, C. G. Bazuin and C. Pellerin, 91 A. Priimagi, J. Vapaavuori, F. J. Rodriguez, C. F. J. Faul, Macromolecules, 2018, DOI: 10.1021/acs.macromol.7b02534. M. T. Heino, O. Ikkala, M. Kauranen and M. Kaivola, Chem. 118 M. Hackel, L. Kador, D. Kropp and H. W. Schmidt, Adv. Mater., 2008, 20, 6358–6363. Mater., 2007, 19, 227–231. 92 C. G. Bazuin and A. Tork, Macromolecules, 1995, 28, 8877–8880. 119 A. Saishoji, D. Sato, A. Shishido and T. Ikeda, Langmuir, 93 Q. Zhang, C. G. Bazuin and C. J. Barrett, Chem. Mater., 2007, 23, 320–326. 2008, 20, 29–31. 120 J. del Barrio, E. Blasco, L. Oriol, R. Alcala´ and C. Sa´nchez- 94 J. Vapaavuori, V. Valtavirta, T. Alasaarela, J.-I. Mamiya, Somolinos, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, A. Priimagi, A. Shishido and M. Kaivola, J. Mater. Chem., 1716–1725. 2011, 21, 15437–15441. 121 J. del Barrio, E. Blasco, C. Toprakcioglu, A. Koutsioubas, 95 M. J. Banach, M. D. Alexander, S. Caracci and R. A. Vaia, O. A. Scherman, L. Oriol and C. Sa´nchez-Somolinos, Chem. Mater., 1999, 11, 2554–2561. Macromolecules, 2014, 47, 897–906. 96 S. Wu and C. Bubeck, Macromolecules,2013,46, 3512–3518. 122 A. Concello´n, E. Blasco, M. Pin˜ol, L. Oriol, I. Dı´ez, 97 N. Zettsu, T. Ogasawara, N. Mizoshita, S. Nagano and C. Berges, C. Sa´nchez-Somolinos and R. Alcala´, J. Polym. T. Seki, Adv. Mater., 2008, 20, 516–521. Sci., Part A: Polym. Chem., 2014, 52, 3173–3184. 98A.V.Medvedev,E.B.Barmatov,A.S.Medvedev,V.P.Shibaev, 123 A. Priimagi, M. Saccone, G. Cavallo, A. Shishido, T. Pilati, S. A. Ivanov, M. Kozlovsky and J. Stumpe, Macromolecules, P. Metrangolo and G. Resnati, Adv. Mater., 2012, 24, 2005, 38, 2223–2229. OP345–OP352.

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124 P. Politzer, J. S. Murray and T. Clark, Phys. Chem. Chem. 146 J. E. Stumpel, M. Saccone, V. Dichiarante, O. Lehtonen, Phys., 2010, 12, 7748–7757. M. Virkki, P. Metrangolo and A. Priimagi, Molecules, 2017, 125F.Fernandez-Palacio,M.Poutanen,M.Saccone,A.Siiskonen, 22, 1844. G.Terraneo,G.Resnati,O.Ikkala,P.Metrangoloand 147 O. Ikkala and G. ten Brinke, Science, 2002, 295, 2407–2409. A. Priimagi, Chem. Mater., 2016, 28, 8314–8321. 148 Y. Zhao, K. Thorkelsson, A. J. Mastroianni, T. Schilling, 126J.Vapaavuori,A.Siiskonen,V.Dichiarante,A.Forni, J. M. Luther, B. J. Rancatore, K. Matsunaga, H. Jinnai, M. Saccone, T. Pilati, C. Pellerin, A. Shishido, P. Metrangolo Y. Wu, D. Poulsen, J. M. Frechet, A. P. Alivisatos and T. Xu, and A. Priimagi, RSC Adv., 2017, 7, 40237–40242. Nat. Mater., 2009, 8, 979–985. 127 Y. Liang, D. Mauran, R. E. Prud’homme and C. Pellerin, 149 A. Sidorenko, I. Tokarev, S. Minko and M. Stamm, J. Am. Appl. Spectrosc., 2008, 62, 941–947. Chem. Soc., 2003, 125, 12211–12216. 128 M. Virkki, O. Tuominen, A. Forni, M. Saccone, 150 B. Nandan, M. K. Vyas, M. Bo¨hme and M. Stamm, Macro- P. Metrangolo, G. Resnati, M. Kauranen and A. Priimagi, molecules, 2010, 43, 2463–2473. J. Mater. Chem. C, 2015, 3, 3003–3006. 151 B. Nandan, E. B. Gowd, N. C. Bigall, A. Eychmu¨ller, 129 O. Kulikovska, L. M. Goldenberg and J. Stumpe, Chem. P. Formanek, P. Simon and M. Stamm, Adv. Funct. Mater., Mater., 2007, 19, 3343–3348. 2009, 19, 2805–2811. 130 F. Lagugne´ Labarthet, T. Buffeteau and C. Sourisseau, 152 P. Hiekkataipale, T. I. Lo¨bling, M. Poutanen, A. Priimagi, J. Phys. Chem. B, 1998, 102, 2654–2662. V. Abetz, O. Ikkala and A. H. Gro¨schel, Polymer, 2016, 107, 131 O. Kulikovska, L. M. Goldenberg, L. Kulikovsky and 456–465. J. Stumpe, Chem. Mater., 2008, 20, 3528–3534. 153 Y. Morikawa, S. Nagano, K. Watanabe, K. Kamata, T. Iyoda 132 T. Ko¨nig, L. M. Goldenberg, O. Kulikovska, L. Kulikovsky, and T. Seki, Adv. Mater., 2006, 18, 883–886. J. Stumpe and S. Santer, Soft Matter, 2011, 7, 4174–4178. 154 T. Seki, J. Mater. Chem. C, 2016, 4, 7895–7910. 133 N. S. Yadavalli, T. Konig and S. Santer, J. Soc. Inf. Disp., 155 A. Kopyshev, C. J. Galvin, J. Genzer, N. Lomadze and

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 2015, 23, 154–162. S. Santer, Langmuir, 2013, 29, 13967–13974. 134 J. E. Koskela, V. Liljestrom, J. Lim, E. E. Simanek, 156 J. Vapaavuori, J. Grosrenaud, C. Pellerin and C. G. Bazuin, R. H. Ras, A. Priimagi and M. A. Kostiainen, J. Am. Chem. ACS Macro Lett., 2015, 4, 1158–1162. Soc., 2014, 136, 6850–6853. 157 S. Roland, C. G. Gamys, J. Grosrenaud, S. Boisse´,C.Pellerin, 135 J. Vapaavuori, A. Priimagi, A. J. Soininen, N. Canilho, R. E. Prud’homme and C. G. Bazuin, Macromolecules,2015, E. Kase¨mi, J. Ruokolainen, M. Kaivola and O. Ikkala, Opt. 48, 4823–4834. Mater. Express, 2013, 3, 711–722. 158 S. Roland, R. E. Prud’homme and C. G. Bazuin, ACS Macro 136 F. Zeng and S. C. Zimmerman, Chem. Rev., 1997, 97, Lett., 2012, 1, 973–976. 1681–1712. 159 H. Yu, H. Liu and T. Kobayashi, ACS Appl. Mater. Interfaces,

This article is licensed under a 137 A. Priimagi, K. Lindfors, M. Kaivola and P. Rochon, ACS 2011, 3, 1333–1340. Appl. Mater. Interfaces, 2009, 1, 1183–1189. 160 H. Liu, T. Kobayashi and H. Yu, Macromol. Rapid Commun., 138 J. Vapaavuori, R. H. A. Ras, M. Kaivola, C. G. Bazuin and 2011, 32, 378–383. A. Priimagi, J. Mater. Chem. C, 2015, 3, 11011–11016. 161 H. Zhang, R. Hao, J. K. Jackson, M. Chiao and H. Yu, Chem. Open Access Article. Published on 23 January 2018. Downloaded 9/25/2021 4:41:32 AM. 139 A. Sobolewska and S. Bartkiewicz, Appl. Phys. Lett., 2012, Commun., 2014, 50, 14843–14846. 101, 193301. 162 D. Chen, H. Liu, T. Kobayashi and H. Yu, J. Mater. Chem., 140 J. Noga, A. Sobolewska, S. Bartkiewicz, M. Virkki and 2010, 20, 3610–3614. A. Priimagi, Macromol. Mater. Eng., 2017, 302, 1600329. 163 S. Wu, J. Huang, S. Beckemper, A. Gillner, K. Wang and 141 A. Sobolewska and S. Bartkiewicz, J. Mater. Chem. C, 2015, C. Bubeck, J. Mater. Chem., 2012, 22, 4989. 3, 5616–5620. 164 T. Alasaarela, D. Zheng, L. Huang, A. Priimagi, B. Bai, 142 E. Schab-Balcerzak, H. Flakus, A. Jarczyk-Jedryka, A. Tervonen, S. Honkanen, M. Kuittinen and J. Turunen, J. Konieczkowska, M. Siwy, K. Bijak, A. Sobolewska and Opt. Lett., 2011, 36, 2411–2413. J. Stumpe, Opt. Mater., 2015, 47, 501–511. 165 T. Ubukata, T. Isoshima and M. Hara, Adv. Mater., 2005, 143 E. Schab-Balcerzak, J. Konieczkowska, M. Siwy, 1630–1633. A. Sobolewska, M. Wojtowicz and M. Wiacek, Opt. Mater., 166 L. M. Goldenberg, V. Lisinetskii and S. Schrader, Adv. Opt. 2014, 36, 892–902. Mater., 2013, 1, 527–533. 144 A. Priimagi, G. Cavallo, A. Forni, M. Gorynsztejn-Leben, 167 S. L. Lim, N.-J. Li, J.-M. Lu, Q.-D. Ling, C. X. Zhu, E.-T. Kang M. Kaivola, P. Metrangolo, R. Milani, A. Shishido, T. Pilati, and K. G. Neoh, ACS Appl. Mater. Interfaces, 2009, 1, 60–71. G. Resnati and G. Terraneo, Adv. Funct. Mater., 2012, 22, 168 T. Mosciatti, S. Bonacchi, M. Gobbi, L. Ferlauto, L. Fabiola, 2572–2579. L. Giorgini, E. Orgiu and P. Samori, ACS Appl. Mater. 145 M. Saccone, V. Dichiarante, A. Forni, A. Goulet-Hanssens, Interfaces, 2016, 8, 6563–6569. G. Cavallo, J. Vapaavuori, G. Terraneo, C. J. Barrett, 169 D. Attianese, M. Petrosino, P. Vacca, S. Concilio, G. Resnati, P. Metrangolo and A. Priimagi, J. Mater. P. Iannelli, A. Rubino and B. Salvatore, IEEE Electron Device Chem. C, 2015, 3, 759–768. Lett., 2008, 29, 44–46.

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